What happened behind the scenes?

I have been watching this show for the first three seasons (I am midway through season 3 now). I see that Peter and Assumpta are soon to leave the show. Ill watch and see how it unfolds. SO I don't need an explanation of what happens on screen.

That said, what was going on behind the scenes that the actors didn't want to continue? Did they just get tired of it? Was there some kind of falling out?

The closest American show I can compare it to is Northern Exposure. When Rob Morrow left it was because he wanted to be in movies. Was it something similar with BallyK?